1. Field of the Invention
The invention relates to an evaluation circuit for call signals in a telecommunication system, and particularly to a circuit for evaluating the level and frequency range of such signals.
2. Description of the Prior Art
DE-SO No. 27 58 542 discloses a telephone exchange call circuit in which the level of an applied call signal is evaluated. To that end the exchange call circuit comprises an opto-coupler circuit connected to a rectifier bridge and to a parallel arrangement of a resistor and a capacitor. The opto-coupler provides electric isolation between the circuit portions effecting the evaluation of the received call signal and the circuit portions connected to the exchange call line. By means of a voltage divider arranged in the output circuit of the opto-coupler the threshold of the call signal level is determined, and an RC-arrangement is used for filtering the rectified call a.c. voltage.
The exchange call circuit defined in the foregoing has the disadvantage that it does not distinguish between frequencies located in the pass region of the low-pass filter (RC-arrangement) or between alternating and direct currents or alternating and direct voltages.
DE-OS No. 33 14 819 discloses a call detection circuit for a telephone subscriber's end station in which evaluation of both the level and frequency of a received call signal is effected. The level evaluation is effected by means of a threshold value switch and the frequency evaluation is effected by time discrimination using counters and an auxiliary clock from a clock source (at a frequency generated in the dialling tone generator).
This call detection circuit has the disadvantage that the circuit arrangement for level evaluation is expensive, an auxiliary clock pulse is required for frequency evaluation, and noise in the subscriber line between the exchange and the subscriber's end station is coupled-in due to switching edges of the signals at the output of the threshold value switch, at the counter output and at the voltage divider outputs.